In a multi-phase brushless DC motor, the rotary field is established by a set of permanent magnets secured to the rotor, and the stationary field is established by sequentially energizing a set of phase windings secured on the stator. A position or back-EMF sensing device detects the position of the rotor and a logic circuit energizes the appropriate stator windings via a pair of bridge inverter switching devices relative to the detected position of the rotor to accelerate the rotor.
There are two distinct purposes for controlling the conduction of the switching devices of a bridge inverter. One purpose is to apply a specific polarity of current to a specific phase winding based on a given rotor magnet position with respect to the phase windings in order to cause the rotor to rotate in a desired direction. This switching, called commutation, occurs at a frequency generally referred to as the electrical frequency of the motor. The second purpose for controlling the conduction of the switching devices of a bridge controller is to regulate the energization current applied to the phase windings by repeatedly applying and removing a constant DC voltage source to the phase windings. By varying the energization current, the speed of the rotor may be varied at some speed less than that at which the rotor would turn if the supply voltage were fully applied to the phase windings. This switching is called pulse width modulation (PWM) switching and occurs at a much higher frequency than the electrical frequency of the motor.
For a brushless DC motor having a trapezoidal back-EMF waveform, the desired phase current waveforms are 120 electrical degree wide current pulses which are of the same polarity and in phase with the flat (constant) portion of the trapezoidal back-EMF waveforms. Thus, for any rotor position, it is desired that only two phases carry current and that the third phase carries zero current. Phases which carry current are referred to as active phases, and phases which do not carry current are referred to as inactive phases. When operating such a variable speed brushless DC motor, both electrical commutation and PWM switching take place concurrently. During the ON period of the PWM switching, the entire DC source voltage is applied across the two active phases to build up the current. During the OFF period of the PWM switching, the active phase current continues to circulate in the active phase windings. Under certain conditions, however, a pulse of current also can flow in the inactive phase winding with a polarity opposite that of the back-EMF, producing a negative or braking torque in the so-called inactive phase. This current flow is undesirable because braking torque lowers the average torque of the motor for a given energization current, making the motor less efficient.